HEART BLOCK TYPES

NORMAL RHYTHM

Now, what if there’s a problem between the SA node and the AV node? What if it happens in the area of the shaded portion? This doesn’t completely block your electrical signal but it does slow it down.

The distance between the P wave and the QRS complex (PR interval) is a little bit longer compared to the normal strip (above). When you have a problem between your SA node and your AV node, but NOT A COMPLETE block, the PR interval becomes too long.

1st DEGREE HEART BLOCK

Now, what happens if you have a problem with your AV node itself (shaded portion)? Again, this isn’t a complete block of the node but what if the AV node is having a hard time keeping up with signals sent from the SA node?

Notice that the PR interval becomes longer and longer and eventually, there’s a P wave but then we don’t have a QRS complex, and then we have another P wave and we have relatively normal looking PR interval. In this situation, it looks like the signal from the P wave gets transmitted to the ventricle as the QRS complex but it’s lagging behind a bit a little each time. It’s like the AV node just can’t quite keep up and eventually it loses step all together and just doesn’t transmit the signal. But then it’s got the chance to rest and get its act together and the next time a P wave comes down, it goes ahead and transmits that to the ventricle as a QRS complex. This is called a second degree block. This is also called a Mobitz Type I and also known as a Wenkebach block.

Mnemonic: Wenke… Weeenke (longer PR)… Weeeeenke (even longer PR)… back (as the heart misses another beat).

2ND DEGREE HEART BLOCK

MOBITZ TYPE I

WENKEBACK BLOCK

Now what happens if you have diseases a little bit lower down (shaded area) just past the AV node? But like before, this isn’t a complete block. Some of the electrical activity is able to get through.

It looks like there are two P waves for every QRS complex. Some of the signal is getting through the ventricle as the QRS complex. Other times the atria are contracting with the P wave but nothing is happening in the ventricle. In this situation, the upper fibers after the AV node are taking too long to reset themselves so they can transmit the signal again. Though, when a signal for instance is transmitted here (circle), we have a normal looking P wave and QRS complex, this fibers are tired and it takes time for them to reset. By the time the next P wave comes along, they’re not ready yet and it takes a little bit longer for them to get ready. By the time the next one comes along, they’re ready to go.

2ND DEGREE HEART BLOCK

MOBITZ TYPE II

What if we take it one step further and completely cut the electrical system? In this case, there is a COMPLETE heart block.

As you can see, the P waves are weird-looking since they sort of just got added on top of a T wave or a QRS complex (sort of overlapping). It looks like the P waves are just marching along and not paying attention to what the QRS complexes are doing. The QRS complex are also marching along pretty regularly just doing their own thing, and there’s a normal T wave after each QRS complex as well – some of them are getting mixed into the P waves. What this means is that this block is not allowing to get from the atria to the ventricles. You might think that the heart will just fail to beat in the ventricles but thankfully, your heart has a backup plan.

Somewhere in the conducting system, some of these cells take over the job of transmitting the initial signal. The signal will START here and it will progress like it normally would, causing the ventricle to contract. These backup generators never do their thing as quickly as they ought to and distance between the QRS complex is much longer than they ought to be, leading to persistently low heart rate.

3rd DEGREE BLOCK

COMPLETE HEART BLOCK